This application claims priority of European Patent Application No. 00302049.2, which was filed on Mar. 14, 2000.
This invention relates to cellular telephone networks and to a method of operating the same.
The background to the related art will be described with reference to a particular application. The reader will readily comprehend that the invention can be applied more generally. When a mobile is switched on in a cellular system, it first searches for the best cell that it can connect to and then attempts to send a message to the best cell found. The contents of this message provide the network with a unique identifier for the mobile, an indication of the quality of the down link (DL) channel and the type (idle/connected) of connection desired, for example. The transmission of the mobile""s initial request for network connection messages is uncoordinated and as a result requires complex procedures, commonly referred to as Random Access CHannel (RACH) procedures, to reduce the probability of interference and collisions between neighbouring mobiles simultaneously attempting to access the network via same cell.
In CDMA cellular systems, the likelihood of a mobile""s RACH message being successfully received by a cell is largely dependent on its transmit power. Unfortunately having all mobiles transmit at the maximum power possible is in many circumstances not only unnecessary but also unacceptable due to the resultant intra-cell interference generated. Several methods of reducing the RACH message transmit have been proposed based on gently ramping up the mobile transmit power of the RACH message.
Methods that reduce the average transmit power of the RACH can be applied to optimise different aspects of a CDMA cellular system. Aspects that can be improved, include uplink cellular capacity and/or cell coverage.
Against this background, there is provided a cellular mobile telephone network, wherein a mobile station requests service in data modulated on a common random access channel (RACH) in a format associated with first a cell in which the mobile station is located, and wherein base stations in cells neighbouring the first cell, are configured to demodulate the RACH message burst having the format associated with the first cell, and to pass the demodulated data to a radio network controller (RNC). As will be explained, this enables the average power required for a successful RACH transmission to be reduced.
In a preferred form, said data is sent in separate preamble and message bursts; and a radio network controller (RNC) is responsive to receipt of the preamble burst, to instruct base stations in the neighbouring cells to demodulate the RACH message burst having the format associated with the first cell, and to pass the demodulated data to a radio network controller (RNC).
In this case, the RNC may be operative to determine from the propagation delay of a data burst from a mobile station located in the first cell, whether the mobile station is in a handover region and, if it is in a handover region, to instruct base stations in neighbouring cells to which the mobile station may move from the handover region, to demodulate the RACH message burst having the format associated with the first cell, and to pass the demodulated data to a radio network controller (RNC) as said.
The mobile station may be operative to compare the signal strengths of transmissions received from different base stations to determine whether or not it is in a handover region, and to send RACH data with one of a reserved set of physical channel attributes if the base station is in the handover region.
In this case the RNC is preferably operative to instruct a base station in the first cell to assign first hardware to search for RACH data having the physical channel attributes belonging to mobile stations located in a handover region, and to assign second hardware to search for RACH data having the physical channel attributes belonging to mobile stations located in a non-handover region.
In that arrangement, the RNC is preferably operative to instruct a base station in a neighbouring cell to assign hardware to search for RACH data having a format associated with the first cell and physical channel attributes belonging to mobile stations located in a handover region.
The invention also extends to a method of operating a cellular mobile telephone network, comprising sending a request for service from a mobile station by means of data modulated on a common random access channel (RACH) in a format associated with first a cell in which the mobile station is located, and at base stations in cells neighbouring the first cell, demodulating the RACH message burst having the format associated with the first cell, and passing the demodulated data to a radio network controller (RNC).
The invention further extends to computer program for carrying out the method.